Over the past several decades numerous types of electrical services have emerged as standard services supplied by utilities. As a result of the variation in service type, many forms of energy meters have also been developed to meter electricity supplied by one or more of these standard services. For example, conventional electromechanical 2-element meters have had two separate and isolated voltage sensing circuits. These two isolated sensing circuits have enabled electromechanical meters to be applied to a variety of electrical services.
In recent years, electronic meters have become widely used and have been replacing electromechanical meters in many installations. While conventional potential transformers may be used in an electronic meter to sense the supplied phase voltages, electronic meters may take advantage of more accurate and lower cost resistive dividers for voltage sensing. However, as described in U.S. Pat. No. 5,548,527 dated Aug. 20, 1996 entitled PROGRAMMABLE ELECTRICAL METER UTILIZING A NON-VOLATILE MEMORY and assigned to the same assignee hereunder, resistive dividers share a common voltage reference and therefore the output signals, i.e., the sensed voltage signals, are not isolated.
In some meter installations, potential transformers external to the meter are used. In these installations the lack of isolation from internal voltage sensors is irrelevant because the external potential transformers provide the required isolation. In meter installations where no external potential transformers are used, the line voltages into the meter are not isolated and 2-element electronic meters with resistive divider voltage sensing cannot be used to meter certain standard electrical services.
FIGS 1A and 1B show common distribution circuits for a 3-wire delta service and a 4-wire delta service, respectively. Each phase of the electrical energy supplied by these services is designated in FIGS 1A and 1B as A, B, and C. The 4-wire delta service includes a neutral or common potential point between phases A and B provided by a center tapped secondary of a transformer. As shown in FIG. 1A, the voltage between each phase of a 3-wire delta circuit is 240 V. The phase voltages for a 4-wire delta circuit may also be 240 V between each phase with 120 V between phase A and neutral, 120 V between phase B and neutral, and 208 V between phase C and neutral. It should be understood that the line-to-line voltages supplied by a 4-wire delta service are not limited to 240 V but may include other standard service voltages such as 480 V.
A common measurement technique used in energy meters to measure power involves multiplying the phase voltage sensed by each of the meter's sensing elements by the associated phase currents. Thus for a 2-element meter, power may be computed as follows: ##EQU1## where V.sub.1 represent the voltage associated with the first element, I.sub.1 represents the current associated with the first element, V.sub.2 represents the voltage associated with the second element, and I.sub.2 represents the current associated with the second element. It should be understood that the equation for real power represents the accumulation of the product terms over a predetermined period of time, e.g., two line cycles. Specifically, sampled voltage and current signals are multiplied and then the results accumulated over some known period of time to arrive at the Watts measurements. Thus the simplified notation, for example, V.sub.1 I.sub.1 as used herein represents the accumulation of that product term over a certain period of time.
For simplicity, reference to power measurements herein will be described in terms of real power. However, it should be understood that equations for apparent as well as reactive power measurements could be substituted for the equation for real power measurements in the description herein.
FIG. 2A is a circle and blade diagram showing the voltage and current connections to a 2-element meter using an isolated voltage sensing technique. The meter socket 1 shown in FIG. 2A is generally referred to as a Form 5S. Electromechanical meters, for example, use potential transformers for voltage sensing. Since potential transformers provide isolation as discussed above, a 2-element electromechanical meter uses a Form 5S type meter socket as shown in FIG. 2A. Accordingly a meter socket 1 has inputs corresponding to V.sub.1, I.sub.1, V.sub.2, and I.sub.2 as depicted in FIG. 2A.
FIG. 2B is a circle and blade diagram showing the voltage and current connections to a 2-element meter in which a non-isolating voltage sensing techniques are used. The meter socket shown in FIG. 2B is referred to as a Form 35S. For example, as discussed above, solid state meters often employ resistive division for voltage sensing. Accordingly, a meter socket 2 has inputs and outputs corresponding to V.sub.1, I.sub.1, V.sub.2, and I.sub.2 as shown in FIG. 2B. Note that meters with non-isolating voltage sensors have their voltage inputs tied together at a common potential 3.
FIG. 3 shows a meter vector diagram for a 3-wire delta service. The phase voltages and currents for phases A, B, and C as shown in FIG. 1A are shown relationally in FIG. 3 mapped to V.sub.1, I.sub.1, V.sub.2, and I.sub.2. FIGS. 4A and 4B show a 2-element meter connected to a 3-wire delta service with external potential transformers and without external potential transformers, respectively. Either type of meter socket 1,2 may be used in connection with a 3-wire delta service. Current transformers 14, 16 are coupled to the meter socket inputs 4, 8, 6, 9 as shown. In FIG. 4A, potential transformers 18, 20 are coupled to the meter socket inputs 22, 24, 10, 12 as shown. The voltage inputs 10,12 are tied together with an external jumper 5. In FIG. 4B, no potential transformers are used external to the meter and therefore, the phase potential from phase A is coupled directly to the meter voltage input 22 for V.sub.1 ; the phase potential for phase B is coupled directly to the meter voltage input 12 for V.sub.2 ; and the phase potential for phase C is coupled directly to the meter voltage input 24 for V.sub.2.
It can be seen by comparing FIG. 3 with FIGS. 4A and 4B, that
V.sub.1 =V.sub.A -V.sub.B PA1 I.sub.2 =I.sub.A PA1 V.sub.2 =V.sub.C -V.sub.B PA1 I.sub.2 =I.sub.C. PA1 V.sub.1 =V.sub.A -V.sub.B PA1 I.sub.1 =I.sub.A -I.sub.B PA1 V.sub.2 =V.sub. C-V.sub.N PA1 I.sub.2 =2I.sub.C PA1 V.sub.1 =V.sub.A -V.sub.B PA1 I.sub.1 =I.sub.A -I.sub.B PA1 V.sub.2 =V.sub.C -V.sub.N PA1 I.sub.2 =2I.sub.C PA1 V.sub.1 =V.sub.A -V.sub.N PA1 I.sub.1 =I.sub.A -I.sub.B PA1 V.sub.2 =V.sub.C -V.sub.N PA1 I.sub.2 =2I.sub.C
Thus, regardless of whether or not external potential transformers are used and regardless of whether or not isolating or non-isolating voltage sensors are used within the meter, i.e., whether a Form 5 or Form 35S is used, the power calculations for a 2-element meter connected to a 3-wire delta service are the same. Consequently, the meter calculations as set forth in equations (1), (2) and (3) above are rather straightforward when connected to a 3-wire delta service.
It is particularly advantageous to use a 2-element meter having resistive dividers with current limiting features as disclosed in U.S. Pat. No. 5,548,527 referred to above in a 3-wire delta installation. Such current limiting features prevent the meter's electronics from operating at elevated levels associated with the service voltage.
It should be noted that the Form 5S meter socket effectively has three voltage inputs or connections when jumper 5 is used to tie together the voltage connections. However, when jumper 5 is not use, the Form 5s has four voltage inputs or connections. The Form 35S effectively has three voltage connections or inputs because the resistors are tied together at common potential point 3 (see FIG. 2B).
FIG. 5 shows a meter vector diagram of a 4-wire delta service. The relationships between the phase voltages and currents for phases A, B, and C from FIG. 1B are shown specifically mapped to measurement voltages V.sub.1 and V.sub.2 and measurement currents I.sub.1 and I.sub.2. FIG. 6 shows a 2-element meter connected to the 4-wire delta service using external potential transformers. As can be seen from FIGS. 5 and 6, the measured parameters remain the same regardless of whether the voltage sensing employed by the meter is isolated or non-isolated, i.e., Form 5S or Form 35S. Because both negative voltage inputs are tied together by jumper 5, the voltage between V.sub.1 in and V.sub.1 out is isolated by external transformer 18 so that a measurement corresponding to V.sub.1 is possible with a Form 35S. The measurement voltages and currents can be defined from FIGS. 5 and 6 thus found as follows:
The line corresponding to phase C is looped through transformer 14 two times so that the measured line current, I.sub.2, is similar in magnitude to the current measured for I.sub.1 which includes the sum of I.sub.A and I.sub.B as shown in FIG. 6.
FIG. 7 shows a 2-element meter with isolating voltage sensors connected to a 4-wire delta service without external potential transformers. The measurement voltages and currents are:
Note that the measurement voltages and currents do not change for a Form 5S when connected to the phase lines with or without external potential transformers.
However, when resistive dividers or other non-isolating voltage sensors are used by the meter, the measurement voltages and currents differ. FIG. 8 shows a 2-element meter with non-isolating voltage sensors connected to a 4-wire delta service without external potential transformers. The meter vector diagram resulting from this connection is shown in FIG. 9. As can be seen from FIGS. 8 and 9, the measurement voltages and currents are:
Since V.sub.1 does not represent the voltage measured between phases A and B, equations (1), (2), and (3) above are not valid for a 4-wire delta service without external potential transformers where the voltage signals from the sensing circuits are not isolated.
As discussed above, resistive dividers are an example of sensing circuits which do not provide isolated output signals because the resistive divider associated with each phase require a common reference potential. Therefore, a 2-element meter having resistive dividers with current limiting as disclosed in U.S. Pat. No. 5,548,527 cannot be used to meter energy supplied by a 4-wire delta service without external potential transformers. Consequently, there exists a need for a 2-element meter with non-isolating voltage sensing circuits, i.e., a Form 35S meter, that is capable of metering energy supplied by a 4-wire delta service.
Moreover, as discussed in co-pending application U.S. Provisional Application No. 60/028,986 filed Oct. 22, 1996 entitled ELECTRONIC ENERGY METER assigned to the same assignee hereunder, there is a general trend to provide versatile electronic energy meters that are capable of metering electrical energy supplied from a variety of services. One advantage of such meters is a reduction of inventory of different meters by utilities. However, use of such meters often require specialized knowledge and training for proper installation. Therefore, there is also a need for a versatile 2-element meter that can be simply installed at installations supplied by a 3-wire delta or a 4-wire delta service, irrespective of whether or not external potential transformers are used.